1. Field of the Invention
This invention relates to a laser control apparatus and a control method which monitors an optical output from multiple laser elements by a photodetector, and controls each laser element by a drive current including a bias current and a modulation current based on a monitored result.
2. Description of the Related Art
In an optical interface for an optical communication system, a laser element (e.g., a laser diode (LD)) may be used as an emitting source. In an optical communication, an optical output of the emitting source should be stabilized to reduce an occurrence of a communication error. However, it is known that an emitting characteristic of the LD is varied easily by ambient temperature. Generally, the optical output of the LD may be stabilized by detecting the optical output of the LD by a photodiode (PD) and controlling a bias current of the LD to keep a light receiving output constant.
However, in some communication methods, the optical output of the LD cannot be stabilized by only controlling the bias current. A modulation circuit which is allowed to adjust a modulation current amplitude in addition to adjusting the bias current is known (e.g., see Japanese Patent Publication JP-A-1994-85363, paragraphs 0008-0013).
Further, a laser array apparatus including integrated multiple LDs may be used as the emitting source for an optical transmission of a multiple channels. In this case, if the optical output of each LD is controlled, a circuit configuration becomes complex.
Consequently, a simplified laser array is disclosed in Japanese Patent JP-B-3130571 (paragraphs 0012-0019, FIG. 1). In this laser array apparatus, one of multiple laser units is set as a reference laser unit, a feedback control of a drive current is performed only about the reference laser unit, and other laser units are controlled by following the control of the reference laser unit's drive current. Consequently, the laser array apparatus is attempted to be simplified.
In this constitution, however, since the optical output of only the reference laser unit of the multiple laser units is monitored, when a variation of a thermal characteristic between the laser units is great, it is difficult to control exactly the laser units other than the reference laser unit.
Therefore, prior to the present invention, there has been no conventional system or method which can provide a laser control apparatus or a control method thereof which can control each optical output of multiple laser elements exactly according to a variation of a thermal characteristic between multiple laser elements or a temperature change.